Refactor overlay rc load in Rust, supporting module modification of rc

[Howard20181]

Modules' rc scripts follow these rules:

1. Ignore all scripts named init.rc.
2. Each script existing in system partitions will be replaced by modules', otherwise, it will concatenated AFTER init.rc, except zygote*.rc which will be ignored (this restriction should be removed by changing the order of injection of zygote rc because it's useful in some use case like enable 32-bit zygote in pixel 7 series).